1. Field of the Invention
The invention disclosed and taught herein relates generally to floating barges used to install a topsides for offshore structures; and more specifically related to systems and methods for stabilizing heave caused by wave action on a barge system during the installation of the topsides.
2. Description of the Related Art
A Spar platform is a type of floating oil platform typically used in very deep waters and is among the largest offshore structures in use. A Spar platform includes a large cylinder or hull supporting a typical rig topsides. The cylinder however does not extend all the way to the seafloor, but instead is moored by a number of mooring lines. Typically, about 90% of the Spar is underwater. The large cylinder serves to stabilize the platform in the water, and allows movement to absorb the force of potential high waves, storms or hurricanes. Low motions and a protected center well also provide an excellent configuration for deepwater operations. In addition to the hull, the Spar's three other major parts include the moorings, topsides, and risers. Spars typically rely on a traditional mooring system to maintain their position.
Installing a deck or topsides to an offshore floating structure has always been a challenge, particularly on deep draft floaters like the Spar, which are installed in relatively deep water. In the past, heavy lifting vessels (“HLV”), including but not limited to, derrick barges have been used for topsides installations. In traditional efforts, the topsides requires multi-lifting, for example five to seven lifts, to install the whole topsides due to the lifting capacity of available HLV and the increasingly larger sizes of topsides. Due to multi-lifting, the steel weight per unity area of the topsides can be higher than that of topsides of fixed platforms installed with a single lifting. If the weight of the topsides is reduced, the weight of the Spar hull to support the topsides may also be reduced. The same principles are applicable to other offshore structures to which a topsides can be mounted.
Recently, catamaran float-over systems have been used to install a topsides onto a Spar platform to resolve the above size challenges. A float-over method is a concept for the installation of the topsides as a single integrated deck onto a Spar hull in which the topsides is loaded and transported with at least two float-over barges to the installation site for the Spar hull. At the installation site, the float-over barges are positioned to straddle the Spar hull with the topsides above the Spar hull, the elevation is adjusted between the topsides and the Spar hull, and the topsides is installed to the Spar hull. Installation of the topsides to the Spar hull by the float-over method can allow a high proportion of the hook-up and pre-commissioning work to be completed onshore prior to installation on the Spar platform, which can significantly reduce both the duration and cost of the offshore commissioning phase. The float-over installation method allows for the installation of the integrated topsides or production deck on a fixed or floating structure without any heavy lift operation.
However, to accomplish the catamaran float-over procedure, the float-over barges are necessarily separated. The separation causes significant load on the barges primarily from the frequency and timing of wave motion on each barge. The vertical movement of the barge from such wave motion is termed “heave.” The heave is greatest on the barges when the wave direction impacts the barge perpendicularly to the longitudinal axis of a typical rectangular barge having a length (bow to stern) significantly greater than its width (beam), known as “beam seas”. Typically, the least heave occurs when the wave direction impacts the barge parallel along the longitudinal axis known as “head seas”, with intermediate heave occurring when the wave direction is at an angle, such as 45 degrees to the longitudinal axis, known as “quartering seas”. Depending on the period (“Tw”) of the wave and therefore distance from crest to crest, one barge can be at a crest of the wave while the other barge is at the trough of the wave, and then the first barge can be at the trough while the other barge is at the crest, as the wave continues to move through the barges.
Similar issues and challenges occur with single barge floatover systems. In single barge systems, the topsides is loaded onto a single barge, the topsides is transported to an installation site on the barge, the barge is typically floated over and between two portions of an offshore structure, and the topsides is installed thereto. The single barge is susceptible to similar heave and differential motion relative to the offshore structure.
With a relative stable offshore structure and a relatively unstable barge affected especially by beam seas, the transfer of the topsides to the offshore structure can be difficult. The heave causes significant differential movement between the topsides and the offshore structure, and complexities in smoothly and efficiently installing the topsides to the offshore structure.
There remains then a need to provide a stabilized barge system for a float-over procedure with a topsides.